Due to advances in communications systems, various types of wireless terminals have been introduced to consumers. Current mobile communications systems have adapted techniques of the 3rd generation partnership project (3GPP) family, such as long term evolution (LTE) and LTE-Advanced. Such mobile communication systems are high-speed and high-capacity communications systems capable of providing a data service for transmitting various types of data, such as images and wireless data, as well as voice-centric services.
For such high-speed and high-capacity communications systems, technology has been required for enhancing capability of a terminal using small cells. For using small cells, technology has been also required for enabling the terminal to configure connections to a plurality of base stations including the small cells. In this regard, there have been many researches conducted for developing dual connectivity (DC) that aggregates one or more cells controlled by two or more base stations. Further, there also have been many researches for processing a large amount of data at a high speech through carrier aggregation (CA) of aggregating one or more cells.
Since a user travels with a terminal, the terminal may get out of an area of a current base station (e.g., cell) and get into a new area of another base station. In this case, it is required to perform a handover procedure for enabling the terminal to transmit and receive data. The handover procedure refers to a procedure enabling the terminal to seamlessly have a data transmission and reception service when the terminal migrates from a first base station to a second base station.
In the related-art handover procedure, however, the terminal must perform a random access procedure to resume data transmission and reception, due to movement from one base station (e.g., cell) to another. In this case, data transmissions may be interrupted for a significant length of time. In addition, in the-related art, while a terminal is releasing a secondary base station (SeNB) to perform a SeNB change, the data transmission interruption may continue for a significant length of time. Such defects may degrade the reliability and the service quality.